Digital control channels having logical channels for multiple access radiocommunication

ABSTRACT

A communications system in which information is transmitted in successive time slots grouped into a plurality of superframes which are, in turn, grouped into a plurality of hyperframes. A remote station is assigned to one of the time slots in each of the superframes for paging the remote station, each hyperframe including at least two superframes, and the information sent in the assigned time slot in one superframe in each hyperfame is repeated in the assigned time slot in the other superframe(s) in each hyperframe. Each superframe can include a plurality of time slots used for sending paging messages to remote stations, grouped into a plurality of successive paging frames, and the time slot to which the remote station is assigned is included once in every paging frame. Also, each superframe may include time slots comprising a logical channel for broadcast control information and time slots comprising a logical paging channel. Information sent in the assigned time slot may direct the remote station to read the broadcast control information, and the information may have been encoded according to an error correcting code and include a plurality of bits having polarities that are inverses of cyclic redundancy check bits produced by the encoding. Also, the broadcast control information may comprise special messages that are included in respective time slots comprising a logical special message channel, the time slots of the special message channel may be grouped in successive SMS frames, and the SMS frames may be synchronized to start with a start of a superframe.

This application is a continuation, of application Ser. No. 08/331,703,is now U.S. Pat. No. 5,604,744 filed Oct. 31, 1994. This application isa continuation in part of U.S. patent application Ser. No. 08/147,254entitled “A Method for Communicating in a Wireless, CommunicationSystem”, which was filed on Nov. 1, 1993 is now U.S. Pat. No. 5,603,081,and which is incorporated in this application by reference. Thisapplication is also a continuation in part of U.S. patent applicationSer. No. 07/956,640 is now U.S. Pat No. 5,404,355 entitled “DigitalControl Channel”, which was filed on Oct. 5, 1992, and which isincorporated in this application by reference.

BACKGROUND

Applicants' invention relates generally to radiocommunication systemsthat use digital control channels in a multiple access scheme and moreparticularly to cellular TDMA radiotelephone systems having digitalcontrol channels.

The growth of commercial radiocommunications and, in particular, theexplosive growth of cellular radiotelephone systems have compelledsystem designers to search for ways to increase system capacity withoutreducing communication quality beyond consumer tolerance thresholds. Oneway to increase capacity is to use digital communication and multipleaccess techniques such as TDMA, in which several users are assignedrespective time slots on a single radio carrier frequency.

In North America, these features are currently provided by a digitalcellular radiotelephone system called the digital advanced mobile phoneservice (D-AMPS), some of the characteristics of which are specified inthe interim standard IS-54B, “Dual-Mode Mobile Station-Base StationCompatibility Standard”, published by the Electronic IndustriesAssociation and Telecommunications Industry Association (EIA/TIA).Because of a large existing consumer base of equipment operating only inthe analog domain with frequency-division multiple access (FDMA), IS-54Bis a dual-mode (analog and digital) standard, providing for analogcompatibility in tandem with digital communication capability. Forexample, the IS-54B standard provides for both FDMA analog voicechannels (AVC) and TDMA digital traffic channels (DTC), and the systemoperator can dynamically replace one type with the other to accommodatefluctuating traffic patterns among analog and digital users. The AVCsand DTCs are implemented by frequency modulating radio carrier signals,which have frequencies near 800 megahertz (MHz) such that each radiochannel has a spectral width of 30 kilohertz (KHz).

In a TDMA cellular radiotelephone system, each radio channel is dividedinto a series of time slots, each of which contains a burst ofinformation from a data source, e.g., a digitally encoded portion of avoice conversation. The time slots are grouped into successive TDMAframes having a predetermined duration. The number of time slots in eachTDMA frame is related to the number of different users that cansimultaneously share the radio channel. If each slot in a TDMA frame isassigned to a different user, the duration of a TDMA frame is theminimum amount of time between successive time slots assigned to thesame user.

The successive time slots assigned to the same user, which are usuallynot consecutive time slots on the radio carrier, constitute the user'sdigital traffic channel, which may be considered a logical channelassigned to the user. As described in more detail below, digital controlchannels (DCCs) can also be provided for communicating control signals,and such a DCC is a logical channel formed by a succession of usuallynon-consecutive time slots on the radio carrier.

According to IS-54B, each TDMA frame consists of six consecutive timeslots and has a duration of 40 milliseconds (msec). Thus, each radiochannel can carry from three to six DTCs (e.g., three to six telephoneconversations), depending on the source rates of the speechcoder/decoders (codecs) used to digitally encode the conversations. Suchspeech codecs can operate at either full-rate or half-rate, withfull-rate codecs being expected to be used until half-rate codecs thatproduce acceptable speech quality are developed. A full-rate DTCrequires twice as many time slots in a given time period as a half-rateDTC, and in IS-54B, each radio channel can carry up to three full-rateDTCs or up to six half-rate DTCs. Each full-rate DTC uses two slots ofeach TDMA frame, i.e., the first and fourth, second and fifth, or thirdand sixth of a TDMA frame's six slots. Each half-rate DTC uses one timeslot of each TDMA frame. During each DTC time slot, 324 bits aretransmitted, of which the major portion, 260 bits, is due to the speechoutput of the codec, including bits due to error correction coding ofthe speech output, and the remaining bits are used for guard times andoverhead signalling for purposes such as synchronization.

It can be seen that the TDMA cellular system operates in abuffer-and-burst, or discontinuous-transmission, mode: each mobilestation transmits (and receives) only during its assigned time slots. Atfull rate, for example, a mobile station might transmit during slot 1,receive during slot 2, idle during slot 3, transmit during slot 4,receive during slot 5, and idle during slot 6, and then repeat the cycleduring succeeding TDMA frames. Therefore, the mobile station, which maybe battery-powered, can be switched off, or sleep, to save power duringthe time slots when it is neither transmitting nor receiving. In theIS-54B system in which the mobile does not transmit and receivesimultaneously, a mobile can sleep for periods of at most about 27 msec(four slots) for a half-rate DTC and about 7 msec (one slot) for afull-rate DTC.

In addition to voice or traffic channels, cellular radiocommunicationsystems also provide paging/access, or control, channels for carryingcall-setup messages between base stations and mobile stations. Accordingto IS-54B, for example, there are twenty-one dedicated analog controlchannels (ACCs), which have predetermined fixed frequencies fortransmission and reception located near 800 MHz. Since these ACCs arealways found at the same frequencies, they can be readily located andmonitored by the mobile stations.

For example, when in an idle state (i.e., switched on but not making orreceiving a call), a mobile station in an IS-54B system tunes to andthen regularly monitors the strongest control channel (generally, thecontrol channel of the cell in which the mobile station is located atthat moment) and may receive or initiate a call through thecorresponding base station. When moving between cells while in the idlestate, the mobile station will eventually “lose” radio connection on thecontrol channel of the “old” cell and tune to the control channel of the“new” cell. The initial tuning and subsequent re-tuning to controlchannels are both accomplished automatically by scanning all theavailable control channels at their known frequencies to find the “best”control channel. When a control channel with good reception quality isfound, the mobile station remains tuned to this channel until thequality deteriorates again. In this way, mobile stations stay “in touch”with the system. The ACCs specified in IS-54B require the mobilestations to remain continuously “awake” (or at least for a significantpart of the time, e.g. 50%) in the idle state, at least to the extentthat they must keep their receivers switched on.

While in the idle state, a mobile station must monitor the controlchannel for paging messages addressed to it. For example, when anordinary telephone (land-line) subscriber calls a mobile subscriber, thecall is directed from the public switched telephone network (PSTN) to amobile switching center (MSC) that analyzes the dialed number. If thedialed number is validated, the MSC requests some or all of a number ofradio base stations to page the called mobile station by transmittingover their respective control channels paging messages that contain themobile identification number (MIN) of the called mobile station. Eachidle mobile station receiving a paging message compares the received MINwith its own stored MIN. The mobile station with the matching stored MINtransmits a page response over the particular control channel to thebase station, which forwards the page response to the MSC.

Upon receiving the page response, the MSC selects an AVC or a DTCavailable to the base station that received the page response, switcheson a corresponding radio transceiver in that base station, and causesthat base station to send a message via the control channel to thecalled mobile station that instructs the called mobile station to tuneto the selected voice or traffic channel. A through-connection for thecall is established once the mobile station has tuned to the selectedAVC or DTC.

When a mobile subscriber initiates a call, e.g., by dialing thetelephone number of an ordinary subscriber and pressing the “send”button on the mobile station, the mobile station traits the dialednumber and its MIN and an electronic serial number (ESN) over thecontrol channel to the base station. The ESN is a factory-set,“unchangeable” number designed to protect against the unauthorized useof the mobile station. The base station forwards the received numbers tothe MSC, which validates the mobile station, selects an AVC or DTC, andestablishes a through-connection for the call as described above. Themobile may also be required to send an authentication message.

It will be understood that a communication system that uses ACCs has anumber of deficiencies. For example, the format of the forward analogcontrol channel specified in IS-54B is largely inflexible and notconducive to the objectives of modem cellular telephony, including theextension of mobile station battery life. In particular, the timeinterval between transmission of certain broadcast messages is fixed andthe order in which messages are handled is also rigid. Also, mobilestations are required to re-read messages that may not have changed,wasting battery power. These deficiencies can be remedied by providing aDCC having new formats and processes, one example of which is describedin U.S. patent application Ser. No. 07/956,640 entitled “Digital ControlChannel”, which was filed on Oct. 5, 1992, and which is incorporated inthis application by reference. Using such DCCs, each IS-54B radiochannel can carry DTCs only, DCCs only, or a mixture of both DTCs andDCCs. Within the IS-54B framework, each radio carrier frequency can haveup to three full-rate DTCs/DCCs, or six half-rate DTCs/DCCs, or anycombination in-between, for example, one full-rate and four half-rateDTCs/DCCs. As described in this application, a DCC in accordance withApplicants' invention provides a further increase in functionality.

In general, however, the transmission rate of the DCC need not coincidewith the half-rate and full-rate specified in IS-54B, and the length ofthe DCC slots may not be uniform and may not coincide with the length ofthe DTC slots. The DCC may be defined on an IS-54B radio channel and mayconsist, for example, of every n-th slot in the stream of consecutiveTDMA slots. In this case, the length of each DCC slot may or may not beequal to 6.67 msec, which is the length of a DTC slot according toIS-54B. Alternatively (and without limitation on other possiblealternatives), these DCC slots may be defined in other ways known to oneskilled in the art.

As such hybrid analog/digital systems mature, the number of analog usersshould diminish and the number of digital users should increase untilall of the analog voice and control channels are replaced by digitaltraffic and control channels. When that occurs, the current dual-modemobile terminals can be replaced by less expensive digital-only mobileunits, which would be unable to scan the ACCs currently provided in theIS-54B system. One conventional radiocommunication system used inEurope, known as GSM, is already an all-digital system, in which200-KHz-wide radio channels are located near 900 MHz. Each GSM radiochannel has a gross data rate of 270 kilobits per second and is dividedinto eight full-rate traffic channels (each traffic time slot carrying116 encrypted bits).

In cellular telephone systems, an air-interface communications linkprotocol is required in order to allow a mobile station to communicatewith the base stations and MSC. The communications link protocol is usedto initiate and to receive cellular telephone calls. As described inU.S. patent application Ser. No. 081047,452 entitled “Layer 2 Protocolfor the Random Access Channel and the Access Response Channel,” whichwas filed on Apr. 19, 1993, and which is incorporated in thisapplication by reference, the communications link protocol is commonlyreferred to within the communications industry as a Layer 2 protocol,and its functionality includes the delimiting, or framing, of Layer 3messages. These Layer 3 messages may be sent between communicating Layer3 peer entities residing within mobile stations and cellular switchingsystems. The physical layer (Layer 1) defines the parameters of thephysical communications channel, e.g., radio frequency spacing,modulation characteristics, etc. Layer 2 defines the techniquesnecessary for the accurate transmission of information within theconstraints of the physical channel, e.g., error correction anddetection, etc. Layer 3 defines the procedures for reception andprocessing of information transmitted over the physical channel.

Communications between mobile stations and the cellular switching system(the base stations and the MSC) can be described in general withreference to FIGS. 1 and 2. FIG. 1 schematically illustrates pluralitiesof Layer 3 messages 11, Layer 2 frames 13, and Layer 1 channel bursts,or time slots, 15. In FIG. 1, each group of channel bursts correspondingto each Layer 3 message may constitute a logical channel, and asdescribed above, the channel bursts for a given Layer 3 message wouldusually not be consecutive slots on an IS-54B carrier. On the otherhand, the channel bursts could be consecutive; as soon as one time slotends, the next time slot could begin.

Each Layer 1 channel burst 15 contains a complete Layer 2 frame as wellas other information such as, for example, error correction informationand other overhead information used for Layer 1 operation. Each Layer 2frame contains at least a portion of a Layer 3 message as well asoverhead information used for Layer 2 operation. Although not indicatedin FIG. 1, each Layer 3 message would include various informationelements that can be considered the payload of the message, a headerportion for identifying the respective message's type, and possiblypadding.

Each Layer 1 burst and each Layer 2 frame is divided into a plurality ofdifferent fields. In particular, a limited-length DATA field in eachLayer 2 frame contains the Layer 3 message 11. Since Layer 3 messageshave variable lengths depending upon the amount of information containedm the Layer 3 message, a plurality of Layer 2 frames may be needed fortransmission of a single Layer 3 message. As a result, a plurality ofLayer 1 channel bursts may also be needed to transmit the entire Layer 3message as there is a one-to-one correspondence between channel burstsand Layer 2 frames.

As noted above, when more than one channel burst is required to send aLayer 3 message, the several bursts are not usually consecutive burstson the radio channel. Moreover, the several bursts are not even usuallysuccessive bursts devoted to the particular logical channel used forcarrying the Layer 3 message. Since time is required to receive,process, and react to each received burst, the bursts required fortransmission of a Layer 3 message are usually sent in a staggeredformat, as schematically illustrated in FIG. 2 and as described above inconnection with the IS-54B standard.

FIG. 2 shows a general example of a forward (or downlink) DCC configuredas a succession of time slots 1, 2, . . . , N, . . . included in theconsecutive time slots 1, 2, . . . sent on a carrier frequency. TheseDCC slots may be defined on a radio channel such as that specified byIS-54B, and may consist, as seen in FIG. 2 for example, of every n-thslot in a series of consecutive slots. Each DCC slot has a duration thatmay or may not be 6.67 msec, which is the length of a DTC slot accordingto the IS-54B standard.

As shown in FIG. 2, the DCC slots may be organized into superframes(SF), and each superframe includes a number of logical channels thatcarry different kinds of information. One or more DCC slots may beallocated to each logical channel in the superframe. The exemplarydownlink superframe in FIG. 2 includes three logical channels: abroadcast control channel (BCCH) including six successive slots foroverhead messages; a paging channel (PCH) including one slot for pagingmessages; and an access response channel (ARCH) including one slot forchannel assignment and other messages. The remaining time slots in theexemplary superframe of FIG. 2 may be dedicated to other logicalchannels, such as additional paging channels PCH or other channels.Since the number of mobile stations is usually much greater than thenumber of slots in the superframe, each paging slot is used for pagingseveral mobile stations that share some unique characteristic, e.g., thelast digit of the MIN.

For purposes of efficient sleep mode operation and fast cell selection,the BCCH may be divided into a number of sub-channels. U.S. patentapplication Ser. No. 07/956,640 discloses a BCCH structure that allowsthe mobile station to read a minimum amount of information when it isswitched on (when it locks onto a DCC) before being able to access thesystem (place or receive a call). After being switched on, an idlemobile station needs to regularly monitor only its assigned PCH slots(usually one in each superframe); the mobile can sleep during otherslots. The ratio of the mobile's time spent reading paging messages andits time spent asleep is controllable and represents a tradeoff betweencall-set-up delay and power consumption.

Since each TDMA time slot has a certain fixed information carryingcapacity, each burst typically carries only a portion of a Layer 3message as noted above. In the uplink direction, multiple mobilestations attempt to communicate with the system on a contention basis,while multiple mobile stations listen for Layer 3 messages sent from thesystem in the downlink direction. In known systems, any given Layer 3message must be carried using as many TDMA channel bursts as required tosend the entire Layer 3 message.

Digital control and traffic channels are desirable for these and otherreasons described in U.S. patent application No. 08/147,254, entitled “AMethod for Communicating in a Wireless Communication System”, which wasfiled on Nov. 1, 1993, and which is incorporated in this application byreference. For example, they support longer sleep periods for the mobileunits, which results in longer battery life. Although IS-54B providesfor digital traffic channels, more flexibility is desirable in usingdigital control channels having expanded functionality to optimizesystem capacity and to support hierarchical cell structures, i.e.,structures of macrocells, microcells, picocells, etc. The term“macrocell” generally refers to a cell having a size comparable to thesizes of cells in a conventional cellular telephone system (e.g., aradius of at least about 1 kilometer), and the terms “microcell” and“picocell” generally refer to progressively smaller cells. For example,a microcell might cover a public indoor or outdoor area, e.g., aconvention center or a busy street, and a picocell might cover an officecorridor or a floor of a high-rise building. From a radio coverageperspective, macrocells, microcells, and picocells may be distinct fromone another or may overlap one another to handle different trafficpatterns or radio environments.

FIG. 3 is an exemplary hierarchical, or multi-layered, cellular system.An umbrella macrocell 10 represented by a hexagonal shape makes up anoverlying cellular structure. Each umbrella cell may contain anunderlying microcell structure. The umbrella cell 10 includes microcell20 represented by the area enclosed within the dotted line and microcell30 represented by the area enclosed within the dashed line correspondingto areas along city streets, and picocells 40, 50, and 60, which coverindividual floors of a building. The intersection of the two citystreets covered by the microcells 20 and 30 may be an area of densetraffic concentration, and thus might represent a hot spot.

FIG. 4 represents a block diagram of an exemplary cellular mobileradiotelephone system, including an exemplary base station 100 andmobile station 120. The base station includes a control and processingunit 130 which is connected to the MSC 140 which in turn is connected tothe PSTN (not shown). General aspects of such cellular radiotelephonesystems are known in the art, as described by the above-cited U.S.patent applications and by U.S. Pat. No. 5,175,867 to Wejke et al.,entitled “Neighbor-Assisted Handoff in a Cellular Communication System,”and U.S. patent application Ser. No. 07/967,027 entitled “Multi-modeSignal Processing,” which was filed on Oct. 27, 1992, both of which areincorporated in this application by reference.

The base station 100 handles a plurality of voice channels through avoice channel transceiver 150, which is controlled by the control andprocessing unit 130. Also, each base station includes a control channeltransceiver 160, which may be capable of handling more than one controlchannel. The control channel transceiver 160 is controlled by thecontrol and processing unit 130. The control channel transceiver 160broadcasts control information over the control channel of the basestation or cell to mobiles locked to that control channel. It will beunderstood that the transceivers 150 and 160 can be implemented as asingle device, like the voice and control transceiver 170, for use withDCCs and DTCs that share the same radio carrier frequency.

The mobile station 120 receives the information broadcast on a controlchannel at its voice and control channel transceiver 170. Then, theprocessing unit 180 evaluates the received control channel information,which includes the characteristics of cells that are candidates for themobile station to lock on to, and determines on which cell the mobileshould lock. Advantageously, the received control channel informationnot only includes absolute information concerning the cell with which itis associated, but also contains relative information concerning othercells proximate to the cell with which the control channel isassociated, as described in U.S. Pat. No. 5,353,332 to Raith et al.,entitled “Method and Apparatus for Communication Control in aRadiotelephone System,” which is incorporated in this application byreference.

As noted above, one of the goals of a digital cellular system is toincrease the user's “talk time”, i.e., the battery life of the mobilestation. To this end, U.S. patent application Ser. No. 07/956,640discloses a digital forward control channel (base station to mobilestation) that can carry the types of messages specified for currentanalog forward control channels (FOCCs), but in a format which allows anidle mobile station to read overhead messages when locking onto the FOCCand thereafter only when the information has changed; the mobile sleepsat all other times. In such a system, some types of messages arebroadcast by the base stations more frequently than other types, andmobile stations need not read every message broadcast.

Also, application Ser. No. 07/956,640 shows how a DCC may be definedalongside the DTCs specified in IS-54B. For example, a half-rate DCCcould occupy one slot and a full-rate DCC could occupy two slots out ofthe six slots in each TDMA frame. For additional DCC capacity,additional half-rate or full-rate DCCs could replace DTCs. In general,the transmission rate of a DCC need not coincide with the half-rate andfull-rate specified in IS-54B, and the length of the DCC time slots neednot be uniform and need not coincide with the length of the DTC timeslots.

Although the above-described communication systems are highly beneficialand are markedly different from previous systems, Applicants'communication system described in this application is optimized toachieve the goal of long sleep times at the same time as the goal ofgood immunity to channel impairments due to noise and interference likeRayleigh channel fading. As an added feature, Applicants' communicationsystem is capable of broadcasting special messages to the mobilestations without affecting other aspects of its performance.

SUMMARY

A radiocommunication system according to Applicants' inventioneliminates several of the drawbacks mentioned above.

In one aspect, Applicants' invention provides a method of communicatinginformation to a remote station which comprises the steps of groupingthe information into a plurality of successive time slots on a radiocarrier signal; grouping the time slots into a plurality of successivesuperframes; grouping successive superframes into successivehyperframes; and assigning the remote station to one of the time slotsin each of the superfames, the assigned slot being used for paging theremote station. In addition, each hyperframe includes at least twosuperframes, and the information sent in the assigned time slot in onesuperframe in each hyperframe is repeated in the assigned time slot inthe other superframe(s) in each hyperframe.

In a further aspects of Applicants' invention, each superframe includesa plurality of time slots used for sending paging messages to remotestations, the time slots used for sending paging messages in successivehyperframes are grouped into a plurality of successive paging frames,and the time slot to which the remote station is assigned is includedonce in every paging frame. Also, each superframe may include time slotscomprising a logical channel for broadcast control information and timeslots comprising a logical paging channel, the assigned time slot beingincluded in the time slots comprising the logical paging channel.Moreover, the information sent in the assigned time slot may includeinformation directing the remote station to read the broadcast controlinformation, and the information may have been encoded according to anerror correcting code and include a plurality of bits having polaritiesthat are inverses of cyclic redundancy check bits produced by theencoding. The remote station, in response to decoding the plurality ofbits, could read the broadcast control information.

In another aspect, the method would further include the step, in theremote station, of decoding the assigned time slot only in firstsuperframes in successive hyperframes if the assigned time slot isproperly decoded. Also, the method could include the step, in the remotestation, of decoding the assigned time slot in the other superframes inthe successive hyperframes if the assigned time slot in the firstsuperframes is not properly decoded. The mobile station would determinethat the assigned time slot is properly decoded based on a plurality ofcyclic redundancy check bits included in the assigned time slot.

In a further aspect, each time slot has a duration of substantially 6.67millisecond, and each superframe consists of thirty-two time slotsdistributed among ninety-six consecutive time slots on the radio carriersignal. Also, the broadcast control information may comprise specialmessages that are included in respective time slots comprising a logicalspecial message channel, the time slots of the special message channelare grouped in successive SMS frames, and the SMS frames aresynchronized to start with a start of a superframe.

In yet a further aspect, Applicants' invention provides a method ofcommunicating information to a remote station comprising the steps ofgrouping the information into a plurality of time slots on a radiocarrier signal; grouping the time slots into a plurality of superframes;and sending, in each slot in each superframe, phase information foridentifying a position of the slot in the superframe. The superframephase information may be a count indicating a time of next occurrence ofa slot including overhead information.

In a further aspect, Applicants' invention provides a method ofcommunicating overhead information to a remote station comprising thesteps of grouping the overhead information into a plurality of timeslots on a radio carrier signal; grouping other information into anotherplurality of time slots; successively transmitting the time slots havingoverhead information and the time slots having other information; andindicating in each time slot the respective time slot's temporalposition with respect to the next transmission of time slots havingoverhead information.

In the method, each time slot's temporal position can be indicated by acount for indicating a time interval until the next transmission of thetime slots having overhead information. Also, the time interval betweensuccessive transmissions of the time slots having overhead informationis at least an order of magnitude greater than a duration of each timeslot. Moreover, the time slots including the overhead information maycomprise a logical channel for broadcast control information and thetime slots including the other information may comprise special messagesthat are included in other time slots comprising a logical specialmessage channel.

In other aspects of the method, the time slots of the special messagechannel are grouped in successive SMS frames, and each SMS frame maycorrespond to a respective one of a plurality of SMS sub-channels. Aspecial message may span at least two SMS frames of a respective SMSsub-channel, and each special message may be encrypted according to arespective encryption method. As an alternative, the special messagesincluded in the time slots of a first one of the SMS sub-channels may beencrypted according to a first encryption method and the specialmessages included in the time slots of at least one other SMSsub-channel may be encrypted according to a second encryption method.

BRIEF DESCRIPTION OF THE DRAWINGS

The features and advantages of Applicants' invention will be understoodby reading this description in conjunction with the drawings, in which:

FIG. 1 illustrates a plurality of Layer 3 messages, Layer 2 frames, andLayer 1 channel bursts in a communication system;

FIG. 2 is a generalized view of a digital control channel (DCC) havingtime slots which are grouped into superframes;

FIG. 3 illustrates a typical multi-layered cellular system employingumbrella macrocells, microcells and picocells;

FIG. 4 represents an exemplary implementation of an apparatus for aradiotelephone system according to the present invention;

FIG. 5 shows a hyperframe structure;

FIG. 6 shows the logical channels of the DCC;

FIG. 7 shows an exemplary TDMA frame structure;

FIGS. 8a-8 c show exemplary slot formats on the DCC;

FIG. 9 shows the partitioning of data before channel encoding;

FIG. 10 shows a paging frame structure;

FIG. 11 shows an SMS frame structure;

FIG. 12 shows an example of SMS sub-channel multiplexing;

FIG. 13a-13 c show F-BCCH Layer 2 frames; and

FIG. 14 is a flowchart describing exemplary methods according to thepresent invention.

DETAILED DESCRIPTION

The following description is in terms of a cellular radiotelephonesystem, but it will be understood that Applicants' invention is notlimited to that environment. Also, the following description is in thecontext of TDMA cellular communication systems, but it will beunderstood by those skilled in the art that the present invention mayapply to other digital communication applications such as Code DivisionMultiple Access (CDMA). The physical channel may be, for example, arelatively narrow band of radio frequencies (FDMA), a time slot on aradio frequency (TDMA), a code sequence (CDMA), or a combination of theforegoing, which can carry speech and/or data, and is not limited to anyparticular mode of operation, access technique, or system architecture.

In one aspect of Applicants' invention, communication between mobilestations and base stations is structured into successions of differentkinds of logical frames. FIG. 5 illustrates the fame structure of aforward (base station to mobile station) DCC and shows two successivehyperframes (HF), each of which preferably comprises a respectiveprimary superframe (SF) and a respective secondary superframe. It willbe recognized, of course, that a hyperframe could include more than twosuperframes.

Three successive superframes are illustrated in FIG. 5, each comprisinga plurality of time slots that are organized as logical channels F-BCCH,E-BCCH, S-BCCH, and SPACH that are descibed in more detail below. Atthis point, it is sufficient to note that each superframe in a forwardDCC includes a complete set of F-BCCH information (i.e., a set of Layer3 messages), using as many slots as are necessary, and that eachsuperframe begins with a F-BCCH slot. After the F-BCCH slot or slots,the remaining slots in each superframe include one or more (or no) slotsfor the E-BCCH, S-BCCH, and SPACH logical channels.

Referring to FIG. 5, and more particularly to FIG. 6, each superframe ofthe downlink (forward) DCC preferably comprises a broadcast controlchannel BCCH, and a short-message-service/paging/access channel SPACH.The BCCH comprises a fast BCCH (the F-BCCH shown in FG. 5); an extendedBCCH (the E-BCCH); and a short-message-service BCCH (the S-BCCH), all ofwhich are used, in general, to carry generic, system-related informationfrom the base stations to the mobiles. The BCCH is unidirectional,shared, point-to-multipoint, and unacknowledged. The SPACH comprises ashort-message-service channel SMSCH, a plurality of paging channels PCH,and an access response channel ARCH, which are used to send informationto specific mobile stations relating to short-message-servicepoint-to-point messages (SMSCH), paging messages (PCH), and messagesresponding to attempted accesses (ARCH) as described below. The SPACH isunidirectional, shared, and unacknowledged. The PCH may be consideredpoint-to-multipoint, in that it can be used to send paging messages tomore than one mobile station, but in some circumstances the PCH ispoint-to-point. The ARCH and SMSCH are generally point-to-point,although messages sent on the ARCH can also be addressed to more thanone mobile station.

For communication from the mobile stations to the base stations, thereverse (uplink) DCC comprises a random access channel RACH, which isused by the mobiles to request access to the system. The RACH logicalchannel is unidirectional, shared, point-to-point, and acknowledged. Alltime slots on the uplink are used for mobile access requests, either ona contention basis or on a reserved basis. Reserved-basis access isdescibed in U.S. patent application Ser. No. 08/140,467, entitled“Method of Effecting Random Access in a Mobile Radio System”, which wasfiled on Oct. 25, 1993, and which is incorporated in this application byreference. One important feature of RACH operation is that reception ofsome downlink information is required, whereby mobile stations receivereal-time feedback for every burst they send on the uplink. This isknown as Layer 2 ARQ, or automatic repeat request, on the RACH. Thedownlink information preferably comprises twenty-two bits that may bethought of as another downlink sub-channel dedicated to carrying, in thedownlink, Layer 2 information specific to the uplink. This flow ofinformation, which can be called shared channel feedback, enhances thethroughput capacity of the RACH so that a mobile station can quicklydetermine whether any burst of any access attempt has been successfullyreceived. Other aspects of the RACH are described below.

The F-BCCH logical channel carries time-critical system information,such as the structure of the DCC, other parameters that are essentialfor accessing the system, and an E-BCCH change flag that is described inmore detail below; as noted above, a complete set of F-BCCH informationis sent in every superframe. The E-BCCH logical channel carries systeminformation that is less time-critical than the information sent on theF-BCCH; a complete set of E-BCCH information (i.e., a set of Layer 3messages) may span several superframes and need not be aligned to startin the first E-BCCH slot of a superframe. The S-BCCH logical channelcarries short broadcast messages, such as advertisements and informationof interest to various classes of mobile subscriber, and possibly systemoperation information, such as change flags for the other logicalchannels. An important aspect of Applicants' invention is that theS-BCCH decouples the system overhead information sent on the F-BCCH andE-BCCH from the broadcast message service (S-BCCH), obtaining maximumsystem flexibility. It would be possible to omit the S-BCCH and send itsmessages on the E-BCCH or even the F-BCCH, but doing so would delay thedelivery of important system information since the SMS messages would beintermingled with the system overhead messages.

As for the SPACH slots, they are assigned dynamically to the SMSCH, PCH,and ARCH channels based on transmitted header information. The SMSCHlogical channel is used to deliver short messages to a specific mobilestation receiving SMS services. The PCH logical channel carries pagingmessages and other orders to the mobiles, such as the F-BCCH change flagdescribed above and in U.S. patent application Ser. No. 07/956,640.Mobile stations are assigned respective PCH slots in a manner describedin more detail below. A mobile station listens to system responses senton the ARCH logical channel upon successful completion of the mobile'saccess on a RACH. The ARCH may be used to convey AVC or DTC assignmentsor other responses to the mobile's attempted access.

An important aspect of Applicants' invention is that every PCH slot inthe primary superframe of a hyperframe is repeated in the secondarysuperframe of that hyperframe. This is called “specification guaranteedrepeat”. Thus, once a mobile station has read the BCCH information, itcan enter sleep mode after determining, based on its MIN or some otherdistinguishing characteristic, which single PCH slot it is to monitorfor a paging message. Then, if the mobile station properly receives apaging message sent in its PCH slot in a primary superframe, the mobilecan sleep through the entire associated secondary superframe, therebyincreasing the life of its batteries. If and only if the mobile stationcannot correctly decode its assigned PCH slot in a primary superframe,the mobile reads the corresponding PCH slot in the associated secondarysuperframe.

It should be understood, however, that the mobile station may read itsPCH slot in only one of the superframes, either primary or secondary,for a variety of reasons, whether or not the slot is correctly decoded.This may be permitted to maximize the mobile's sleep time. Also, afterthe mobile has read its PCH slot in one of the superframes (for example,a primary superframe), the mobile may monitor other control channelsduring at least part of the time until the next (primary) superframewithout missing a page on the first control channel. Indeed, the mobilemay even read a paging slot on another control channel. This enablescell reselection to be carried out smoothly and avoids the mobile'sbeing blind to pages during such reselection. It will be recognized thatreselection is facilitated when the two control channels aresynchronized, at least to the extent that a time offset between theirsuperframes is known, which is information that may be provided on theE-BCCH for example.

One aspect of a DCC as described in U.S. patent application Ser. No.071956,640 is that the F-BCCH slots in successive superframes carry thesame information until change flags transmitted in the PCH slots toggle,or otherwise change value in a predetermined way. This feature is alsoprovided in the systems and methods described in this application. Also,the E-BCCH and S-BCCH information may span both superframes in ahyperframe, and even several hyperframes, which represents a tradeoffbetween BCCH bandwidth (i.e., the member of slots needed for sending acomplete set of BCCH messages) and the time required for a full cycle ofmessages sent. The toggling of a change flag in the PCH slot indicatesthat new data will be found on the F-BCCH sent in the followingsuperframe. In this way, once a mobile station has read the BCCHinformation on a DCC, the mobile need awaken only to read its assignedPCH slot; when the change flag in its PCH slot toggles, the mobilelearns that it must either awaken or stay awake to re-acquire theF-BCCH, which has changed; if the mobile determines that the change flaghas not toggled, it is not necessary for the mobile to read the F-BCCH.This also increases the mobile's sleep time, and battery life.

In a similar way, the F-BCCH slots may include E-BCCH change flagsindicating that the system has changed the E-BCCH information. Inresponse to an E-BCCH change flag, the mobile would stay awake to readthe E-BCCH slots. It will be understood that the change of the E-BCCHchange flag in the F-BCCH slots is “new data” to be found on the F-BCCHthat would be indicated by the F-BCCH change flag transmitted in the PCHslots. The mobile station preferably stores the value of the E-BCCHchange flag transmitted in the F-BCCH slots before reading the E-BCCH.After the mobile station has acquired the relevant information (whichmay be dependent on the specific task the mobile is engaged in), themobile reads the E-BCCH change notification flag again. The process ofupdating/initiating the E-BCCH message set can be considered successfulwhen the E-BCCH change flag is the same before and after the mobilereads the E-BCCH.

Among the other important features of Applicants' invention, is thatinformation is not interleaved among successive slots, although asdescribed below, information may be interleaved among fields in the sameslot. Also as described below, the downlink information isadvantageously encoded by error correction codes for immunity to channelimpairments, for example a convolutional rate-½ code. It is desirablenot to use “too much” encoding like a convolutional rate-¼ code,however, because the number of user data bits sent in any given channelburst would be low. Also, such encoding is not needed because the BCCHinformation is repeated in every superframe and certain transactions canuse ARQ. The BCCH and PCH cannot use ARQ, of course, but using a singletype of coding is advantageous because it reduces equipment complexity.Therefore, to obtain sufficient protection, somewhat less encoding iscombined with the time diversity provided by specification guaranteedrepeat for the PCH. This combination is also beneficial for sleep modeperformance.

The combination of these features results in a communication system thathas good immunity to errors at the same time that it permits, onaverage, long mobile sleep times. It will be appreciated that theguaranteed repeats of the PCH slots provide time diversity, yielding animproved immunity to errors due to Rayleigh fading that is provided inprevious systems by rate-¼ encoding and inter-burst interleaving. (Ofcourse, specification guaranteed repeat is not an option for speechslots.) Applicants' combination of these features, however, results in acommunication system that permits a mobile that has successfully decodedits PCH slot in a primary superframe to sleep through all of the PCHslots in the corresponding secondary superframe. It will be recognizedthat the a mobile's assigned PCH slots are temporally separated by manytimes the duration of such a slot (6.67 msec).

The BCCH information sent in one or more slots of the DCC comprisesinformation about the serving system and the desired behavior of themobile station when operating in this system. The overhead informationwould include, for example, indications of the following: (1) the pagingslot to which the mobile station is assigned; (2) whether the mobilestation is allowed to make and receive any calls through this basestation or is restricted to only emergency calls; (3) the power level tobe used for transmitting to this base station; (4) the identity of thesystem (home system or visited system); (5) whether or not to use anequalizer for compensating distortion and attenuation effects of theradio channel on the tamitted signal; and (6) the location of other DCCs(frequencies, time slots, time offsets of other DCCs' superframes withrespect to superframes of current DCC) of neighboring base stations. ADCC of a neighboring base station may be selected because the DCC signalreceived from this base station is too weak or for some other reason,e.g., the signal from another base station is stronger than the signalfrom this base station.

When a mobile station locks onto the DCC, the mobile station first readsthe overhead information to determine the system identity, callrestrictions, etc.; the locations of the DCCs of the neighboring basestations (the frequencies, time slots, etc., on which these DCCs may befound); and its paging slot in the superframe (the DCC slot assigned tothe paging frame class to which the mobile station belongs). Therelevant DCC frequencies are stored in memory, and the mobile stationthen enters sleep mode. Thereafter, the mobile station “awakens” onceevery hyperframe, depending on the mobile's paging frame class, to readthe assigned paging slot, and then returns to sleep.

The F-BCCH information transmitted in every superframe allows a mobilestation to read other information in the superframe, to access thesystem, or to quickly find the best serving cell, when first lockingonto a DCC. For example, certain basic information about the low-layerstructure of the DCC must be read by a mobile station before any otherinformation in the superframe can be read. This basic informationincludes, for example, a superframe period (number of DCC slots),whether the DCC is half-rate or full-rate, the DCC format (which slot(s)in a TDMA frame), the location of other BCCH channels, the location ofthe assigned PCH, and whether the mobile station receiver should use anequalizer. Other types of information should also be sent rather oftenso that a mobile station can quickly accept or reject a particular DCC.For example, information about the availability and data capability of acell (the cell may be available only to a closed user group or may notbe capable of handling data transmissions from a mobile station), theidentity of the system and the cell, etc., may be sent in everysuperframe. For accelerating system accesses, it would be sufficient fora mobile station to read only system access rules sent on the F-BCCH.

The E-BCCH is assigned a system-controlled, fixed number of slots ineach superframe, but a long cycle, or set of messages, sent on theE-BCCH may span several superframes; hence, the number E-BCCH slots ineach superframe can be much less than the number of slots needed tocarry the long cycle, or set of messages. If there are not enough E-BCCHslots in a superframe to accommodate all E-BCCH messages, subsequentsuperframes are used. Mobile stations are notified through the F-BCCH asdescribed above of the number and location of E-BCCH slots assigned ineach superframe. A start-of-E-BCCH marker may be sent in the currentF-BCCH (or S-BCCH) to inform the mobile stations that the currentsuperframe contains the start of an E-BCCH message.

With the E-BCCH, long and/or sporadic information may be sent on the DCCwithout affecting the organization of the superframe, e.g., PCHassignments, or the DCC capacity. For example, the list of DCCs ofneighboring base stations may be sent on the E-BCCH. Such a list can berather large, including the locations of, say, ten other DCCs. Such alist would require several slots to transmit, and these slots may bespread out over the E-BCCH of several superframes instead of taking up alarge portion of one superframe. In this way, BCCH overhead is tradedoff for a larger number of paging slots (and consequent increased pagingcapacity).

LAYER 1 FORMAT

An exemplary organization of the information transmitted on each radiochannel, i.e., the channel bursts, or time slots, in accordance withApplicants' invention is shown in FIG. 7. This organization is similarto that specified by the IS-54B standard. The consecutive time slots ona radio channel are organized in TDMA frames of six slots each and TDMAblocks of three slots each so that a plurality of distinct channels canbe supported by a single radio carrier frequency. Each TDMA fame has aduration of 40 msec and supports six half-rate logical channels, threefull-rate logical channels, or various combinations between theseextremes by interchanging one full-rate channel and two half-ratechannels as indicated in the following table. Each slot has a durationof 6.67 msec and carries 324 bits (162 symbols), which have positions ineach slot that are conventionally consecutively numbered 1-324.

Number of Slots Used Slots Rate 1 1 half 2 1,4 full 4 1,4,2,5 2 full 61,4,2,5,3,6 3 full

As explained above, each superframe comprises a predetermined number ofsuccessive time slots (full-rate) of a DCC. Since a complete set ofF-BCCH information is sent in each superframe and since the first slotof each superframe is a F-BCCH slot, each superframe is the intervalbetween such initial F-BCCH slots. It is currently preferred that eachsuperframe consist of thirty-two such time slots, which are distributedamong the logical channels F-BCCH, E-BCCH, S-BCCH, and SPACH asillustrated in FIG. 5 for example. Thus, the duration of each logicalsuperframe is simply 32 TDMA blocks/superframe * 20 msec/TDMA block=640msec, which spans 96 consecutive physical time slots on the radiochannel.

It will be appreciated that this selection represents a balance ofseveral factors that Applicants' currently deem most useful. Forexample, using thirty-two slots, which is an integer power of two,simplifies the implementation of various counters in existing hardwarethat is based on binary signal processing. Also, using thirty-two-slotsuperframes balances call set-up delay against paging channel (and otherchannel) capacity. For a given amount of BCCH information to betransmitted, using longer superframes would increase paging capacity,but would also increase the average set-up delay; using shortersuperframes would decrease the average set-up delay to an extent, butwould also decrease paging capacity and devote a greater proportion ofeach superframe to overhead information. Different balances can be stuckthat would nevertheless fall within the spirit of Applicants' invention.

In order to locate each time slot in each superframe and thus providethe enhanced sleep capabilities made available by Applicants' invention,a superframe phase (SFP) count, which increments by one for eachfull-rate DCC slot in a given superframe, is included as part of theinformation broadcast in each downlink DCC slot. The SFP value sent inthe first slot (an F-BCCH slot) of each superframe may be assigned thevalue 0; the next slot of the same logical DCC is assigned an SFP valueof 1, etc. Thus, for a system using superframes of thirty-two slotseach, the SFP value increments modulo-32, and the SFP value sent in eachslot requires five bits. For a half-rate DCC, only half of the values(e.g., 0, 2, 4, . . . , 30) need be used to identify the slots in eachsuperframe of the DCC.

It will be appreciated that such a modulo-32 up-counter could bereplaced by a modulo-32 down-counter, and for a communication systemthat does not employ superframes having a fixed number of time slots,the modulo-32 up-counter would be replaced by a down counter forindicating the next occurrence of the F-BCCH, or other desired overheadinformation. It is only necessary for the information in a slot toinclude some indication of that slot's position in time with respect tothe next occurring time slot carrying the important overheadinformation. It is also desirable that the information indicate thestart of the superframe/hyperframe/paging-frame structures, i.e., thatthe boundaries of the frame structures all be synchronized with the nextoccurring time slot carrying the important overhead information, butsuch synchronization is not necessary.

Two possible formats for the information sent in the slots of thereverse DCC are shown in FIGS. 8a and 8 b, and a preferred informationformat in the slots of the forward DCC is shown in FIG. 8c. Theseformats are substantially the same as the formats used for the DTCsunder the IS-54B standard, but new functionalities are accorded to thefields in each slot in accordance with Applicants' invention. In FIGS.8a-8 c, the number of bits in each field is indicated above that field.

In general, messages (Layer 2 user data bits) to be carried by the slotsare mapped onto the two DATA fields sent in each slot, and in thedownlink slots, encoded SFP values are sent in the CSFP fields thatuniquely identify each slot according to each slot's relative positionin its superframe. Also in the downlink slots, the BRI, R/N, and CPEfields contain the information used in the random access scheme forLayer 2 ARQ on the RACH; comparable Layer 2 ARQ fields could be includedin the uplink slots. In the forward DCC (FIG. 8c), the DATA fields total260 bits in length, the CSFP field carries twelve bits, and the BRI,R/N, CPE fields for shared channel feedback total twenty-two bits. Inthe reverse DCC, the DATA fields total either a normal 244 bits inlength (FIG. 8a) or an abbreviated 200 bits (FIG. 8b).

The bits sent in the G, R, PREAM, SYNC, SYNC+, and AG fields are used ina conventional way to help ensure accurate reception of the CSFP andDATA fields, e.g., for synchronization, guard times, etc. For example,the SYNC field would be the same as that of a DTC according to IS-54Band would carry a predetermined bit pattern used by the base stations tofind the start of the slot. Also, the SYNC+ field would include a fixedbit pattern to provide additional synchronization information for thebase stations, which would set their receiver gains during the PREAMfield so as to avoid signal distortion.

Referring again to FIG. 8c, the CSFP field in each DCC slot conveys theSFP value that enables the mobile stations to find the start of eachsuperframe. The SFP values are preferably encoded with a (12,8) code,similar to the way the DVCC is encoded according to the IS-54B standard;thus, the CSFP field is preferably twelve bits in length, and theunencoded SFP consists of eight bits. Encoding the SFP values in thisway has the advantage of using the hardware and software already presentin the mobile phone for handling the DVCC. Also, the four check bits arepreferably inverted, enabling a mobile to use the information sent inthe CSFP field to discriminate between a DCC and a DTC since the CSFP ofa DCC and the CDVCC of a DTC have no common codewords. Other ways todiscriminate DCCs from DTCs are described in U.S. patent applicationSer. No. 08/147,254. In view of the importance of the SFP to theoperation of the system, a mobile station might decode the CSFPs inseveral slots in order to ensure accuracy since the CSFP in anyindividual slot is less well protected by encoding and time diversitythan the Layer 3 message in the DATA fields.

When each superframe includes thirty-two slots, the three mostsignificant bits in each eight unencoded SFP bits may be set to zero. Itwill be appreciated that the unused SFP bits could be used forparticular purposes, e.g., to handle superframes consisting of more thanthirty-two slots each or for Layer 1 power control messages. Also, thethree unused SFP bits could be used, either alone or in combination withother unused (reserved) bits transmitted in each slot, for increasingthe redundancy or strengthening the error correction coding of the SFP,if determined to be necessary. It will be appreciated that the SFPinformation could be included in the Layer 2 frame header information,rather than in separate Layer 1 fields as shown.

Also, in a system using thirty-two-slot superframes, it is currentlypreferred that the sixteen CRC, or check, bits in the Layer 2 framessent in the BCCH slots are inverted, while the sixteen check bits in theLayer 2 frames sent in the SPACH slots are not inverted. Using the checkbits in this way is advantageous in some situations where it isnecessary to re-assign a mobile station to another paging slot. Forexample, if a system has been using twelve slots of a thirty-two-slotsuperframe for the BCCH and wants to use thirteen slots for the BCCH,mobile stations assigned to the first paging slot after the BCCH slotsmust be informed that they should monitor another paging slot. Themobiles could obtain this information by decoding one or two bits thatwould identify the type of slot being decoded, but at a cost of reducedbandwidth. In Applicants' system, the mobile stations will recognizethat something has changed when they spot the inverted CRC bits, and inresponse they will re-read the F-BCCH, including the new DCC structuremessage.

A hyperframe count and a primary SF indicator are also preferablyincluded in the information carried by the BCCH slots; in particular asdescribed in more detail below, these information elements are includedin the DCC structure message carried by the F-BCCH. The hyperframe countidentifies which hyperframe of a higher-level structure of paging framesand SMS frames is currently being broadcast, as described below inconnection with FIG. 10. In accordance with Applicants' invention, fourpaging frame classes and/or four broadcast SMS sub-channels may beprovided as described below. The primary superframe indicator is asingle bit that toggles to indicate whether the current superframe isthe primary or the secondary superframe in the current hyperframe; whenits value is zero, the current superframe may be the primary, and viceversa. In one embodiment of Applicants' invention, the hyperframe countcounts modulo-12.

FIG. 9 shows a currently preferred partitioning of the Layer 2 user databits before channel encoding. The DATA fields in the logical channelsBCCH, SPACH, and RACH (normal and abbreviated) preferably use ½-rateconvolutional encoding; thus, the two DATA fields in each forward DCCslot carry 109 plaintext, or unencoded, BCCH or SPACH bits; and the twoDATA fields in each reverse DCC slot carry either a normal 101 plaintextRACH bits or an abbreviated 79 plaintext RACH bits. Also, the encodeduser data bits are preferably interleaved between the two DATA fields ineach slot, but they are not interleaved among DATA fields in differentslots in order to enable the longer sleep times available fromApplicants' invention. Interleaving may be done according to suitableconvenient matrices, like those used under the IS-54B standard.

Different DCCs may be assigned to different radio channel frequencies,and a different number of slots may be allocated to the BCCH on eachDCC. Layer 2/3 information may also be different for each DCC, but thisis not required. In an embodiment in which each DCC includes its ownBCCH, much information is redundant from DCC to DCC, resulting in a lossof paging capacity. In another embodiment, DCCs may be organized inmaster-slave relationships, in which full BCCH information would beavailable only on the master DCC; a mobile monitoring a slave DCC wouldacquire its BCCH information by changing to its slave's correspondingmaster DCC. It is currently preferred that each frequency carry a fullset of BCCH information and a mobile station always acquire all its BCCHinformation on the same frequency as its assigned PCH channel.

The structure of the DCC transmitted on the F-BCCH in the first slot ofeach superframe is the most important information for a mobile toacquire. An advantageous DCC structure message comprises the informationelements listed in the following table.

Information Element I E Type Bit Length Message type M 8 Number ofF-BCCH slots M 2 Number of E-BCCH slots M 3 Number of S-BCCH slots M 4Number of Skipped slots M 3 E-BCCH change notification flag M 1Hyperframe count M 4 Primary superframe indicator M 1 Number of DCCslots on this frequency M 2 MAX_SUPPORTED_PFC M 2 PCH_DISPLACEMENT M 3Additional DCC frequencies O 23-114 Total = 33-147 M = Mandatory O =Optional

As described above, the mobile station normally monitors only one of thePCH slots in a superframe to minimize power consumption, or batterydrain. Since some paging messages may be longer than the capacity of asingle time slot, each PCH slot carries a PCON bit that may be set tocause the assigned mobile station to read additional SPACH slots, thenumber of which is advantageously indicated by a parameterPCH_DISPLACEMENT sent on the F-BCCH. The additional slots to be readpreferably are separated by at least 40 msec (one TDMA frame) from theassigned PCH slot for both full- and half-rate DCCs. For example, for afull-rate DCC, the mobile station would attempt to read every otherSPACH slot up to the number indicated by the PCH_DISPLACEMENT parameter.This is advantageous in that it reduces the trunking loss caused by thecreation of the several distinct paging channels. Also, using everyother SPACH slot in this way gives a mobile station time for processingits received information to determine whether it must read additionalslots. If every SPACH slot were used instead of at least every otherone, a mobile station having a slow processing unit might not completeprocessing by the time the next SPACH slot occurred; since the mobilewould not yet be aware that the PCON bit was set, it would have to readthe next slot even if that were unnecessary and sleep mode performancewould suffer.

Also, the transmission of long ARCH or SMSCH messages to a first mobilestation may be interrupted to allow for the transmission of messages toa second mobile station. Each interruption of an ARCH or SMSCH messageby another SPACH message may be limited to no more than a predeterminednumber n of time slots, or by Layer 3 timeout for SMSCH or ARCHmessages. It will be understood that Layer 3 timeout refers to thecommon practice of waiting for a response to a Layer 3 message only fora predetermined period. The number of interruptions each mobile stationmay suffer may also be limited.

Ordinarily, the probability of a successful transmission of a Layer 3message is inversely related to the length of the message. Since theprobability can be quite small for long messages, a simple-minded systemwould spend much of its time re-transmitting or re-reading entiremessages that were not properly received. In Applicants' system, Layer 3paging and broadcast SMS messages are mapped onto Layer 2 frames, andthese are organized in structures called paging frames and SMS frames,respectively. For the BCCH, if a Layer 2 frame is not received properly,it is not necessary to re-read the entire Layer 3 message but only theimproperly received Layer 2 frame. The ARCH and RACH can use ARQ.

In accordance with an aspect of Applicants' invention, the superframesand hyperframes on each DCC are grouped into a succession of pagingframes, each of which includes an integer number of hyperframes and is amember of one of a plurality of paging frame classes; hence, the PCHslots have the paging frame structure. In accordance with one aspect ofApplicants' invention, the mobile station reads its assigned PCH slotonly in the hyperframes of its allocated paging frame class. (Asdescribed above, each mobile station is allocated a specific PCHsubchannel within a paging frame based preferably on the mobile's IS-54BMIN identity.) In many cases, mobile stations would be allocated apaging frame class that would cause the mobiles to read their assignedPCH slots in each hyperframe; this minimizes call set-up time and sleepduration. But other paging classes would have the mobiles read PCH slotsin more widely separated hyperframes, delaying call set-ups butincreasing sleep times to as much as 123 seconds for some types ofpaging frame structure. Thus, it will be appreciated that PCH slots areincluded in every superframe but the PCH slot assigned to a given mobilemay not be.

Referring to the exemplary table shown in FIG. 10, primary and secondaryPCH slots p and s in the primary and secondary superframes,respectively, of each hyperframe may be grouped in one of four PFclasses PF₁-PF₄, which are distinguished by how frequently the PCH slotinformation is repeated. Class PF₁ may be called the “lowest” PF classbecause PCHs in this class repeat their information with the lowestduration between repeats; in FIG. 10, the PCH slot is repeated in eachsuccessive hyperframe (i.e., in every successive superframe). Class PF₄may be called the “highest” PF class because PCHs in this class repeattheir information with the highest duration between repeats; in FIG. 10,the PCH slot is repeated only every fourth hyperframe. As descibedabove, the PCH information in a primary superframe is guaranteed to berepeated in the corresponding secondary superframe. In FIG. 10 forpaging frame class PF(i), where i=2, 3, 4, only the PCH assignmentswhich are aligned to HF₀ are shown for illustration purposes.

In the embodiment illustrated by FIG. 10, there are only four pagingframe classes that are linearly related, yielding maximum sleep times ofeight superframes, or 5.12 seconds. Longer sleep times can be obtainedby providing more classes that are exponentially related. For example,sleep times of 123 seconds are obtained in a system having eight pagingframe classes in which the delays double from class to class. It will beunderstood that long sleep times can result in access delays that areunacceptable for typical telephone use; for example, most callersattempting to reach a mobile would be unwilling to wait 123 secondsafter dialing the mobile's number for contact to be established.Nevertheless, such delays may be tolerable in some cases, such as remotepolling of equipment like soft-drink dispensers.

In an embodiment using the table illustrated in FIG. 10, the leastcommon multiple of the indices of the four paging frame classes istwelve; this is the reason that the HF counter counts modulo-12, asdescribed above.

Three other terms used in describing the operation of the PF classes aredefault PF class, assigned PF class, and current PF class. The defaultPF class is the class assigned to the mobile station when itssubscription to the system is entered. If the default PF class happensto be higher than the highest class supported by a DCC, as defined bythe parameter MAX_SUPPORTED_PFC in the DCC structure message, the mobilewould use the PF class defined by MAX_SUPPORTED_PFC. The assigned PFclass refers to a PF class assigned to the mobile by the system, forexample in the system's response to a registration request by themobile. The PF class actually used during a communication may be calledthe current PF class.

In another aspect of Applicants' invention, the S-BCCH slots insuccessive superframes are grouped into a succession of fixed-length SMSframes, each preferably consisting of twenty-four superframes (twelvehyperframes) as shown in FIG. 11. This S-BCCH frame structure enablesmessages to be sent with highly variable periodicity without sacrificingcapacity, and as described below, it avoids requiring the mobilestations to re-read constantly the entire S-BCCH information when onlyone of the many messages sent has changed. Also, choosing an SMS framestructure that is conveniently related to the paging frame classstructure enables counters that are already in use for one purpose(paging) to be re-used for another purpose (SMS broadcast messaging).

The SMS frames are advantageously divided into a plurality ofsub-channels, each having its own repetition cycle defined in terms ofunits of possible SMS frames. For most practical situations, thesub-channel repetition time should not be too long. In a manner similarto the handling of the F-BCCH change flag described above, a mobilestation is informed of a change in the contents of particularsub-channels through an SMS transition flag (TF) included in its PCHslot information.

Currently, four SMS sub-channels are preferred, and the SMS sub-channelsare sub-multiplexed on the S-BCCH channel in units of SMS frames, e.g.,SMS frame SMS(i), where i=1, . . . , N, as illustrated in FIG. 12. Itwill be understood that each (Layer 1) time slot carries a respectiveSMS frame and that a Layer 3 SMS message can span several SMS frames.

An SF number is advantageously derived from the hyperframe count andprimary superframe indicator sent on the BCCH as follows:

SF number=2*HF count+primary SF indicator.

The first S-BCCH slot(s) within each SMS frame (superframe 0) wouldcontain a header that describes the structure of the SMS sub-channel. Asnoted above, the number of superframes within each SMS frame is fixed,and thus the number of slots assigned to the SMS frame are 0, 24, 48,72, . . . (full-rate), depending on how many slots per superframe areassigned to S-BCCH. The SMS frame is aligned to start at HF counterequal to zero and in a primary superframe to help the mobile synchronizeto the SMS frame structure. In this way, SMS frames are synchronized tothe hyperframes and superframes, although it will be appreciated thatthe start of an SMS frame is offset from the start of a hyperframe (or aprimary superframe) since the S-BCCH slots are not the first slots in asuperframe. Furthermore, regardless of how many paging frame classes aresupported, the system increments the hyperframe count to provide SMSframe synchronization information to the mobile station.

According to Layer 2 information found in every first slot in each SMSframe, the set of messages in an SMS frame SMS(i) may span a number M(i)of SMS frames before a cycle is completed. Regardless of varying messageset cycles among the sub-channels, SMS frame SMS(i) is always followedby SMS frame SMS((i+1) mod N+1) in order of transmission. Thus, Layer 3broadcast SMS messages can span several SMS frames, which represents atradeoff between the number of slots in each superframe devoted to SMSbroadcast and the time needed for message transmission.

A transition flag (TF) is provided for each SMS sub-channel, and theflags for all SMS sub-channels are submultiplexed onto a single flag,transmitted on the SPACH channel, that points to the next logical SMSframe to be read. For example, FIG. 12 shows flag TF(2) points to SMSframe SMS(2). If the transition flag for a sub-channel indicates achange, the mobile station reads an S-BCCH header field at the start ofthe next logical SMS frame to obtain further information, as describedmore fully below

Header information describes the sub-channeling of the broadcast SMSchannel and is provided in the first slot of every SMS frame. The mobilecan also find the Layer 3 structure of the SMS frame associated withthis header. A suitable SMS Header information element located at thestart of every SMS frame is shown in the table below.

Information Element Range (Logical) Bits Number of Sub-channels 1-4 2Sub-channel Number 1-4 2 Phase Length of Sub-ch. Cycle 1-64 6 PhaseNumber of Sub-ch. Cycle 1-64 6 Number of SMS Messages (N) 1-64 (set to 1plus value in 6 field) ∘ SMS Message ID (Note 1) 0-255 (unique ID incycle) 8 ∘ Layer 2 Frame Start (Note 1) 0-255 (Layer 2 frame identifier)8 Note 1: N instances of these two elements are sent consecutively.

SMS data may span several SMS frames, but the flags TF enableinterruption of the sub-channel cycles (cycle clearing). For example,after a flag TF, the mobile station assumes that the next sub-channel isthe start of the new cycle. There are two ways to change the dataprovided on the broadcast SMS: changing the Layer 3 messages within theSMS (messages may be added and/or deleted from any position in thecycle), and changing the structure of the sub-channels.

The SMS Message IDs, of which there are a set of 256, and theirassociated Layer 2 Frame Starts comprise a list of all messagesappearing in an SMS frame. SMS Message IDs are unique for each SMS frameand the whole set of 256 values is used before the set begins to be usedagain in order to aid the mobile in searching for changed message(s) andin avoiding reading messages that have not changed. A Layer 2 FrameStart information element is provided to point to the start of the Layer2 frame in which the associated SMS message begins (the message does nothave to begin at the start of the Layer 2 frame). A description ofmessage delivery is provided in the description of the S-BCCH Layer 2Protocol given below.

In the example shown in the table below, four messages make up SMS frame1, and it may be assumed that only one slot in each superframe isdedicated to S-BCCH. (Since it is currently preferred that each SMSframe include twenty-four superframes, there are twenty-four slots ineach SMS frame.)

Previous SMS New SMS Frame 1 Header Frame 1 Header Number ofsub-channels 3 Number of sub-channels 3 Sub-channel number 1 Sub-channelnumber 1 Length of sub-ch. cycle 2 Length of sub-ch. cycle 2 Phase ofsub-ch. cycle 1 Phase of sub-ch. cycle 1 Number of SMS messages (N) 4Number of SMS messages (N) 5 ∘1 SMS message ID 1 ∘1 SMS message ID 1 ∘1Layer 2 Frame Start 1 ∘1 Layer 2 Frame Start 1 ∘2 SMS message ID 2 ∘2SMS message ID 2 ∘2 Layer 2 Frame Start 2 ∘2 Layer 2 Frame Start 2 ∘3SMS message ID 3 ∘4 SMS message ID 4 ∘3 Layer 2 Frame Start 2 ∘4 Layer 2Frame Start 2 ∘4 SMS message ID 4 ∘5 SMS message ID 5 ∘4 Layer 2 FrameStart 3 ∘5 Layer 2 Frame Start 3 ∘6 SMS message ID 6 ∘6 Layer 2 FrameStart 3

In the table above, the mobile is assumed to be monitoring the SPACHwhen the TF toggles to indicate a change in the S-BCCH. The mobile knowsfrom its own internal superframe count where the start of the SMS frameis, and it can determine that SMS subchannel three is currently beingbroadcast by reading the SMS header and that the TF points to a changein SMS sub-channel one. When SMS sub-channel one begins, the mobilereads the SMS header. It determines that message 3 is removed; that theposition of message 4 has changed (but the message ID is the same so themobile does not need to re-read this message); and that new messages 5and 6 have been added and must be read. The mobile may skip theappropriate number of Layer 2 frames to read the new messages.

S-BCCH LAYER 2 PROTOCOL

The S-BCCH Layer 2 protocol is used when a TDMA burst carries S-BCCHinformation. Each S-BCCH Layer 2 protocol frame is constructed to fit ina 125-bit envelope. An additional five bits are reserved for use as tailbits, which are the last bits sent to the channel coder, resulting in atotal of 130 bits of Layer 2 information carried within each S-BCCHslot. As noted above, the Layer 2 protocol for S-BCCH operation supportsonly unacknowledged operation. Several different S-BCCH Layer 2 framesare shown in FIGS. 13a, 13 b, 13 c.

FIG. 13a shows a mandatory minimum S-BCCH BEGIN frame and FIG. 13b showsanother S-BCCH BEGIN Frame used when two Layer 3 messages are includedin the frame with the second Layer 3 message being continued in afollowing frame. The BEGIN frames are used for starting the delivery ofone or more Layer 3 messages on the S-BCCH, and it is currentlypreferred that an S-BCCH BEGIN frame be used as the first frame of theS-BCCH cycle. If the first Layer 3 message is shorter than one S-BCCHframe, a begin/end indicator BE is added to the end of the L3DATA fieldas shown to indicate whether or not an additional Layer 3 message isstarted within the BEGIN frame. As shown in FIG. 13a, if the BEindicator is set to indicate “END”, the rest of the BEGIN frame ispadded with FILLER, e.g., zeroes. As shown in FIG. 13b, if the BEindicator is set to indicate “BEGIN”, a new Layer 3 message is startedin the BEGIN frame. If the L3DATA field ends on an S-BCCH frameboundary, the BE indicator is not included in the frame; an “END”indication is implied. If the L3DATA field ends with less than nine bitsremaining in the S-BCCH frame, the BE indicator is set to “END”, and therest of the fame is padded with FILLER.

FIG. 13c shows an S-BCCH CONTINUE Frame (mandatory minimum), which isused for continuation of a Layer 3 message that was too long to fit intothe previous frame. The continuation length indicator CLI fieldindicates how many bits of the CONTINUE frame belong to the continuedmessage, and thus the preceding Layer 3 message may have to be paddedwith FILLER. If the BE indicator is set to “END”, the rest of theCONTINUE frame is padded with FILLER. If the BE indicator is set to“BEGIN”, a new Layer 3 message is started in the CONTINUE frame. If theL3DATA field ends on an S-BCCH frame boundary, the BE indicator is notincluded in the frame; an “END” indication is implied. If the L3DATAfield ends with less than nine bits remaining in the S-BCCH frame, theBE indicator is set to “END”, and the rest of the frame is padded withFILLER.

The CLI makes it possible for mobile stations to receive any messagestarting in a continuation frame, even if the preceding logical framewas not received. The following table summarizes the fields included inthe S-BCCH Layer 2 protocol frames.

Field Name Bit Length Values SCS = S-BCCH Cycle Start 1 0 = Not thestart of an S-BCCH cycle 1 = Start of an S-BCCH cycle BC = Begin /Continue 1 0 = Begin 1 = Continue CLI = Continuation Length 7 Number ofbits remaining in Indicator previous Layer 3 message. L3LI = Layer 3Length 8 Variable length Layer 3 Indicator messages supported up to amaximum of 255 octets L3DATA = Layer 3 Data Variable Contains a portion(some or all) of the Layer 3 message having an overail length indicatedby L3LI. The portion of this field not used to carry Layer 3 data isfilled with zeroes. BE = Begin / End 1 0 = Beginning 1 = End FILLER =Burst Filler Variable All filler bits are set zero CRC = CyclicRedundancy 16 Same generator polynomial as Code IS-54B. The nominal DVCCis applied in the calculation of CRC for each S-BCCH Layer 2 frame.

Similar logical frames can be defined for the F-BCCH and E-BCCH, asdescribed in U.S. patent application Ser. No. 08/147,254 for example,but these are beyond the scope of this application.

LAYER 3 MESSAGES

The S-BCCH Layer 3 messages that are mapped to the Layer 2 frames aredescribed below. In all messages shown in tabular form below, theinformation elements in the top rows of the tables are preferably thefirst elements to be delivered to Layer 2. In the information elements,the most significant bit (the left-most bit in the tables) is the firstbit to be delivered to Layer 2. The information elements are describedin alphabetical order after the description of the messages below.

There are two types of S-BCCH messages used for SMS broadcast: SMS frameheader messages; and SMS non-header messages, which are those used totransfer the actual messages to the mobile stations.

The SMS frame header messages describe the structure of the SMSsub-channel, and are provided in the first slot of each SMS frame. Theformat of a suitable SMS frame header message is described in thefollowing table.

Information Element Type Bit Length Message Type M 8 Number ofSub-channels M 2 Sub-channel Number M 2 Phase Length of Sub-ch. Cycle M6 Phase Number of Sub-ch. Cycle M 6 Number of SMS Messages (N) M 6 ∘ SMSMessage ID (Note 1) M 8 ∘ Layer 2 Frame Start (Note 1) M 8 Total = 46NOTE 1: N instances of these two elements are sent consecutively.

The format of a suitable SMS non-header, broadcast message is asfollows:

Information Element Type Bit Length Message Type M 8 SMS Message ID M 8Text Message Data Unit M N*8 N max. = 253

In one aspect of Applicants' invention, SMS messages may be encrypted ina way that supports different classes of message service, much likecable television systems distinguish premium classes of service from abasic service class by scrambling or otherwise protecting the premiumprogramming. For example, three classes might be provided as follows: abasic class in which any subscriber paying an appropriate fee would beable to de-crypt some of the SMS broadcast messages, such as productadvertisements, weather and vehicle traffic announcements; a higherclass in which a subscriber paying a higher fee would be able tode-crypt the SMS broadcast messages available to the basic class andadditional messages, such as news items; and a highest class in which asubscriber paying a highest fee would be able to de-crypt all of the SMSbroadcast messages, including financial quotations and higher-valueitems of information.

The de-cryption of the SMS messages could be carried out by theprocessing units in the mobile stations according to any of a widevariety of cryptographic techniques. Preferably, each broadcast messagewould include as an attribute an indicator for determining whichencryption key or algorithm should be used to decode the respectivemessage. Such attributes might be included in the SMS frame headers, andthe encryption keys or algorithms could be sent to the mobiles over theair or entered directly, via a “smart card”, for example. As analternative, the subchannels could be individually encrypted, so thatbroadcast SMS messages included in the time slots of one of the SMSsub-channels are encrypted according to one encryption method and thebroadcast SMS messages included in the time slots of another SMSsub-channel are encrypted according to a another encryption method.

INFORMATION ELEMENT DESCRIPTION

A few coding rules apply to the information element descriptions. Forexample, information elements of the type “flag” have values of 0 toindicate “disable”, or “off”, or “false”, and values of 1 to indicate“enable”, or “on”, or “true”. Also, certain BCCH fields do NOT trigger atransition in the BCCH change flag in the SPACH; those fields aredesignated as non-critical, or “NC”. Information elements of the type“transition” are modulo-1 counters for indicating changes in currentstatus. The channel number is coded in accordance with the IS-54Bstandard, unless otherwise noted. All lengths are specified in bits,unless otherwise noted.

Layer 2 Frame Start

This variable indicates the number of slots from the start of the SMSsub-channel cycle to the beginning of the SMS message, which may notbegin in the indicated SMS slot but may be contained in an end/beginburst used to start delivery of this message.

Message Type

This 8-bit information element identifies the function of the messagebeing sent. The message types are coded as follows:

S-BCCH Messages Code (binary-hex) Broadcast Information Message 00100111-27

Number of SMS Messages

This variable indicates the number of broadcast SMS messages in this SMSframe (1 plus the value in this field).

Number of Subchannels

This variable indicates the number of SMS sub-channels being used bythis DCC (1 plus the value in this field).

Phase Length of Sub-ch. Cycle

This variable indicates the number of SMS frames that make up one cycle(1 plus the value in this field).

Phase Number of Sub-ch. Cycle

This variable indicates which SMS frame in the cycle is currently beingbroadcast.

Sub-channel Number

This variable indicates which sub-channel is currently being broadcast.

Since current mobile stations operating under the IS-54B standard sleepfor periods on the order of only a few milliseconds, the electroniccircuits that constitute the mobile station's processing unit wouldpreferably be optimized to take complete advantage of the long sleeptimes made available by Applicants' invention. To a somewhat lesserextent, current mobile station transceivers and other electroniccomponents would also benefit from optimization for longer sleep times.In addition, the processing units would be expected to support thehigher data throughput on a DCC.

According to exemplary embodiments of the present invention, methods forgrouping and transmitting information can be described by way of theflowchart illustrated in FIG. 14. Therein, at block 1400, information isgrouped into a plurality of time slots. The grouped time slots are then,at block 1410, further grouped into superframes. The superframes mayinclude logical channels such as, for example, the BCCH, PCH and SMSchannels. After grouping, the time slots are transmitted at block 1420,including superframe phase information as described above.

It is, of course, possible to embody the invention in specific formsother than those described above without departing from the spirit ofthe invention. The embodiments descibed above are merely illustrativeand should not be considered restrictive in any way. The scope of theinvention is determined by the following claims, rather than thepreceding description, and all variations and equivalents which fallwithin the scope of the claims are intended to be embraced therein.

What is claimed is:
 1. A method of communicating information to a remotestation comprising the steps of: grouping the information into aplurality of successive time slots on a radio carrier signal; groupingthe time slots into a plurality of successive superframes; and groupingthe successive superframes into a plurality of successive hyperframes,wherein at least two successive superframes are grouped into eachhyperframe; wherein each superframe includes at least one time slotcomprising a logical channel for broadcast control information and atleast one time slot comprising a logical paging channel, wherein thelogical channel for broadcast control information also includes alogical special message channel.
 2. The method of claim 1, wherein atleast one time slot of the special message channel are grouped insuccessive short message service (SMS) frames, and the SMS frames aresynchronized with respective hyperframes.
 3. The method of claim 2,wherein each SMS frame corresponds to a respective one of a plurality ofSMS sub-channels.
 4. The method of claim 3, wherein a special messagespans at least two SMS frames of a respective SMS sub-channel.
 5. Themethod of claim 3, wherein the special messages included in the timeslots of a first one of the SMS sub-channels are encrypted according toa first encryption method and the special messages included in the timeslots of at least one other SMS sub-channel are encrypted according toanother encryption method.
 6. The method of claim 3, wherein eachspecial message is encrypted according to a respective encryptionmethod.
 7. A method of communicating information to a remote stationcomprising the steps of: grouping the information into a plurality oftime slots; grouping the time slots into a plurality of superframes; andsending, in each slot in each superframe, superframe phase informationfor identifying a position of the slot in the superframe, wherein thesuperframe phase information is twelve bits in length encoded from eightbit phase information.
 8. The method of claim 7, wherein the superframephase information is a count indicating a time of next occurrence of aslot including overhead information.
 9. The method of claim 7, whereinthe count is encoded according to a predetermined error correcting code,polarities of a plurality of cyclic redundancy check bits produced byencoding the count are inverted, and the bits having inverted polaritiesare included in the respective slot.
 10. The method of claim 7, whereinthe eight bits include five bits of superframe phase information andthree other bits.
 11. A method of communicating information to a remotestation comprising the steps of: grouping the information into aplurality of time slots; grouping the time slots into a plurality ofsuperframes; and sending, in each slot in each superframe, a count foridentifying a position of the slot in the superframe, wherein the countis encoded according to a predetermined error correcting code,polarities of a plurality of cyclic redundancy check bits produced byencoding the count are inverted, and the bits having inverted polaritiesare included in the respective slot.
 12. A method of communicatingoverhead information to a remote station comprising the steps of:grouping the overhead information into a plurality of time slots on aradio carrier signal; grouping other information into another pluralityof time slots; successively transmitting the time slots having overheadinformation and the time slots having other information; and indicatingin each time slot the respective time slot's temporal position withrespect to a start of the next transmission of time slots havingoverhead information, wherein a time interval between successivetransmissions of the time slots having overhead information is at leastan order of magnitude greater than a duration of each time slot.
 13. Amethod of communicating overhead information to a remote stationcomprising the steps of: grouping the overhead information into aplurality of time slots on a radio carrier signal; grouping otherinformation into another plurality of time slots; successivelytransmitting the time slots having overhead information and the timeslots having other information; and indicating in each time slot therespective time slot's temporal position with respect to a start of thenext transmission of time slots having overhead information, wherein thetime slots including the overhead information comprise a logical channelfor broadcast control information and the time slots including the otherinformation comprises special messages that are included in other timeslots comprising a logical special message channel.
 14. The method ofclaim 13, wherein the time slots of the special message channel aregrouped in successive SMS frames.
 15. The method of claim 14, whereineach SMS frame corresponds to a respective one of a plurality of SMSsub-channels.
 16. The method of claim 15, wherein a special messagespans at least two SMS frames of a respective SMS sub-channel.
 17. Themethod of claim 15, wherein the special messages included in the timeslots of a first one of the SMS sub-channels are encrypted according toa first encryption method and the special messages included in the timeslots of at least one other SMS sub-channel are encrypted according toanother encryption method.
 18. The method of claims 15, wherein eachspecial message is encrypted according to a respective encryptionmethod.
 19. A remote station for receiving information sent in aplurality of successive time slots on a radio carrier signal comprising:a receiver for receiving the radio carrier signal; means for processingthe information in the time slots on the received radio carrier signal,wherein the processing means reads, in each time slot in a superframe, acount indicating a next time of occurrence of a slot including overheadinformation, wherein the count is eight bits which has been encoded to atwelve bit field.
 20. The remote station of claim 19, wherein the countis encoded according to a predetermined error correcting code,polarities of a plurality of cyclic redundancy check bits produced byencoding the counts are inverted and the bits having inverted polaritiesare included in the respective slot.
 21. A base station comprising: aprocessor for; (a) grouping the information into a plurality ofsuccessive time slots on a radio carrier signal; (b) grouping the timeslots into a plurality of successive superframes; and (c) grouping thesuccessive superframes into a plurality of successive hyperframes,wherein at least two successive superframes are grouped into eachhyperframe; wherein each superframe includes at least one time slotcomprising a logical channel for broadcast control information and atleast one time slot comprising a logical paging channel, wherein thelogical channel for broadcast control information also includes alogical special message channel; and a transceiver for transmitting saidhyperframes as a control channel.
 22. The base station of claim 21,wherein at least one time slot of the logical special message channelare grouped in successive short message service (SMS) frames, and theSMS frames are synchronized with respective hyperframes.
 23. The basestation of claim 22, wherein each SMS frame corresponds to a respectiveone of a plurality of SMS sub-channels.
 24. The base station of claim23, wherein a special message spans at least two SMS frames of arespective SMS sub-channel.
 25. The base station of claim 23, whereinthe special messages included in the time slots of a first one of theSMS sub-channels are encrypted according to a first encryption methodand the special messages included in the time slots of at least oneother SMS sub-channel are encrypted according to another encryptionmethod.
 26. The base station of claim 23, wherein each special messageis encrypted according to a respective encryption method.
 27. A basestation comprising: a processor for: (a) grouping information into aplurality of time slots; and (b) grouping the time slots into aplurality of superframes; and a transceiver for sending, in each slot ineach superframe, a count for identifying a position of the slot in thesuperframe, wherein the count is encoded according to a predeterminederror correcting code, polarities of a plurality of cyclic redundancycheck bits produced by encoding the count are inverted, and the bitshaving inverted polarities are included in the respective slot.
 28. Abase station comprising: a processor for: (a) grouping overheadinformation into a plurality of time slots on a radio carrier signal;and (b) grouping other information into another plurality of time slots;and a transceiver for successively transmitting the time slots havingoverhead information and the time slots having other information,wherein in each time slot the respective time slot's temporal positionis indicated with respect to a start of the next transmission of timeslots having overhead information, wherein a time interval betweensuccessive transmissions of the time slots having overhead informationis at least an order of magnitude greater than a duration of each timeslot.
 29. A base station comprising: a processor for: (a) groupingoverhead information into a plurality of time slots on a radio carriersignal; and (b) grouping other information into another plurality oftime slots; and a transceiver for successively transmitting the timeslots having overhead information and the time slots having otherinformation, wherein in each time slot the respective time slot'stemporal position is indicated with respect to a start of the nexttransmission of time slots having overhead information, wherein the timeslots including the overhead information comprise a logical channel forbroadcast control information and the time slots including the otherinformation comprises special messages that are included in other timeslots comprising a logical special message channel.
 30. The base station23, wherein the time slots of the special message channel are grouped insuccessive SMS frames.
 31. The base station of claim 30, wherein eachSMS frame corresponds to a respective one of a plurality of SMSsub-channels.
 32. The base station of claim 31, wherein a specialmessage spans at least two SMS frames of a respective SMS sub-channel.33. The base station of claim 31, wherein the special messages includedin the time slots of a first one of the SMS sub-channels are encryptedaccording to a first encryption method and the special messages includedin the time slots of at least one other SMS sub-channel are encryptedaccording to another encryption method.
 34. The base station of claim31, wherein each special message is encrypted according to a respectiveencryption method.